The conversion of methane in valuable functionalised products constitutes one of the great challenges to modern Chemistry and, in particular, the catalytic synthesis of acetic acid from that gas and carbon monoxide has attracted a current high interest. For this process, a few catalysts based on vanadinum oxides or heteropolyacids have been recently found [1], whereas a lower catalytic activity or selectivity has been recognized for other systems of Pd(OAc)2Cu(OAc)2 [2], CaCl2 [3], NaVO3 [4], RhCl3 [5,6] (in the presence of O2 [6] with formation also of formic acid and methanol), lanthanide salts [7], K2S2O8 [8] or superacids [9].
Moreover, the synthesis of carbonylated products without requiring the use of the noxious carbon monoxide is also of high interest and recently the conversion of methane into methyl trifluoroacetate or methyl acetate has been achieved by using vanadium heteropolyacids [10] or Cu(OAc)2 [11] catalysts. Alternative processes for the conversion of methane into acetic acid by carbonylation of the former by carbon dioxide are also known in heterogeneous catalysis at temperatures in the 100-500° C. range, with Pd [12], Rh [13), Ir [13], Ru [13] or Cu/Co [14,15] catalysts, and they can involve two distinct stages with methanol as an intermediate [16].
The invention under consideration aims to select catalysts and establish a process for the single-pot direct conversion of methane into acetic acid, preferably without the use of carbon monoxide as the carbonylating agent, under mild or moderate temperature and pressure conditions.
Such catalytic systems would present high advantages, in terms of simplicity and energy saving, over the industrial process that is currently followed and involves three distinct complicated and energy expensive stages, i.e. (i) the steam reforming of methane (highly endothermic process, catalysed by a metal catalyst) to form the “synthesis gas”, (ii) the catalytic conversion of this gas, also at high temperature, in methanol, and (iii) the carbonylation of this alcohol by carbon monoxide to give acetic acid, usually by the Monsanto process which requires an expensive catalyst (based on rhodium or iridium in a BP-Amoco modified route). The invention in analysis uses a catalyst of vanadium, a much cheaper metal than those above.